Liquid Crystal (LC) displays (LCD) and lenses (LCL) are known in the art. In large majority of cases that use LCs, an electrically variable gradient index (so-called GRIN) optical lens is formed by controlling the relative orientation gradient of the LC molecules in space within the clear aperture (CA) of the device. Then, this molecular orientation being sensitive to the electrical field, the gradient (and respectively, the optical power of the LCL) may be changed by changing the electrical stimulus parameters (voltage, frequency or their combination) without any macroscopic mechanical movement or deformation.
A variety of LCL designs have been proposed that control the orientation of the LC molecules in response to a spatially non uniform electric field, see for example the review of S. SATO, Applications of Liquid Crystals to Variable-Focusing Lenses, OPTICAL REVIEW Vol. 6, No. 6 (1999) 471-485. One, seemingly simple, approach is the use of multiple electrode arrays (such as used in LCDs) to generate the lens-like electric field profile in space. However, the complexity of its manufacturing and of its dynamic control reduces its attractiveness and industrial acceptance.
Another approach was described (see S. SATO above), that uses a combination of a hole-patterned electrode (HPE) and a transparent uniform electrode (TUE), FIG. 1a (hereafter the LC alignment layers and other standard elements of LC cell will be omitted to simplify the drawings. Also schematic cross-sectional views of various designs will be mainly presented for the same reasons). The application of an electrical potential difference on those two electrodes will generate spatially non uniform electric field (between points ACB, as illustrated schematically in the FIG. 1b). Thus, if the electrical potential distribution (and the corresponding electric field) have the appropriate spatial profile then the corresponding reorientation of LC molecules and the refractive index modulation within the CA of the device may have the desired spherical (or aspherical, see hereafter) form enabling thus a good quality lens. The optical power (OP) of such a lens (measured in Diopters) may be expressed as OP=2 L Δn/r2, where L is the thickness of the LC layer, Δn is the difference of refractive index in the center (around the point C) and at the periphery (around the points A and B) of the lens and r is the radius of the clear aperture. Note that the focal distance F (measured in meters) is the inverse of OP, F=1/OP. This design being much simpler to manufacture still has some important drawbacks. Namely, the distance (defined by the LC thickness L+ the thickness of the top substrate H) between the HPE and TUE must be relatively large to ensure smooth spatial profile of the electric field inside the LC layer. This electrode separation necessarily increases the voltages (several tens of volts) required for the control of the LCL.
In an article published by A. F. Naumov et al., entitled “Liquid-Crystal Adaptive Lenses with Modal Control”, OPTICS LETTERS/Vol, 23, No. 13/Jul. 1, 1998, an LCL configuration was proposed (shown in FIG. 2), which uses an HPE that is inside of the LC cell (the top substrate of the Sato design, shown in FIG. 1, being flipped at 180°, upside down). In this case, the HPE and TUE are very close to each other (separated just by the LC of thickness L) and a few volts (<5V) are enough for the control of the OP of the LCL. However, the electric field profile would have an abrupt character inside of the LC layer here without specific solutions. This is the reason why a high resistivity or weakly conductive layer (WCL) is cast on the surface of the HPE that smoothens the above mentioned field profile thanks to the very high sheet resistance Rs, which is defined as R=(dσ)−1, where d is the thickness of the WCL and σ is its conductivity. This smoothening phenomenon may be presented by using the concept of attenuation of the electric potential (when going from the periphery to the center of the HPE) in a classical electronic RC circuit where the capacitance of the unit area is defined by the combination of two electrodes (the TUE and the HPE that is covered by a WCL) containing the dielectric LC layer in between. At the same time, the role of the electrical resistance R is mainly played by the sheet resistance Rs of the WCL.
It happens that the “RC factor” of miniature cameras (with CA at the order of 1.5 mm to 2 mm) and the dielectric properties of the LC layer ∈LC and its thickness L are such that the sheet resistance Rs of the WCL, that is necessary for smooth electric field profile, is in the range of tens of MΩ/. The fabrication of such films is an extremely difficult task since such electrical properties correspond to the transition (often called “percolation”) zone. In addition, the consumer product cameras are supposed to work with unpolarized light. This requires the use of two LC layers (with their molecules being oriented in perpendicular planes, shown in FIG. 3a) to handle two cross oriented polarizations of unpolarized light. To have two such “half” lenses focusing in the same way, we need to have two WCLs of the same Rs (within ≈±3%). This imposes very severe conditions on the manufacturing of a polarization independent “full” lens, given that the repeatability of this sheet resistance is very poor, as demonstrated in FIG. 3b. There is another fundamental limitation to this approach: the attenuation of the electrical potential (when going from the periphery of the HPE, points A or B, towards the center of the lens, the point C) is defined by the physical nature of the RC circuit and is very difficult to control and obtain specific aspherical profiles, which are required to have good optical image quality. Finally, all materials with appropriate Rs values (that we know so far) are very sensitive to temperature variations.
Several alternative approaches have been developed to address, at least partially, the problems of Naumov's geometry. One of them (proposed by LensVector 1) is the use of a single WCL to eliminate the severe requirements of manufacturing repeatability, shown in FIG. 4. In this configuration, the HPE and the WCL are positioned between two substrates (almost symmetrically) which serve as bottom and top substrates for two cross oriented LC layers. Thus, the same control electrode structure (HPE+WCL) is used to drive both LC layers similarly.
An alternative approach (shown in FIG. 5) was proposed by B. Wang, M. Ye, M. Yamaguchi, and S. Sato, Thin Liquid Crystal Lens with Low Driving Voltages, Japanese Journal of Applied Physics 48 (2009) 098004. In this geometry, the WCL layer is close to the LC layers, while there is an additional electrically connected disc shaped electrode (DSE) in the middle of HPE, both being positioned outside of the LC cell. While this approach helps to avoid transitory molecular orientational defects (so-called disclinations) and to achieve a better control of the profile of electric field, it still suffers from several drawbacks, including the problem of manufacturability of the WCL. This is because, two WCLs (one for each LC layer) are still needed here or only one WCL and control electrode (HPE+DSE) may be used but it must be positioned relatively far from the TUE since now two LC layers must be driven by the same HPE+DSE structure). In addition, here there is a need to have two independent continuously variable voltages V1 and V2 to drive the LCL. Thus, the grounding of the TUE and applying the voltage V1 on the HPE may create a spatially non uniform potential distribution and corresponding optical power (as shown schematically by the solid curve in FIG. 5b). At the same time, the application of the voltage V2 to the DSE may generate a uniform electrical potential (as demonstrated by the solid and dashed horizontal lines, shown in FIG. 5b) avoiding thus the appearance of disclinations or allowing the continuous control of the optical power and aberrations of the lens. The absolute values of those voltages also are still higher than those used in the Naumov's approach (because of the additional distance between electrodes imposed by the thickness H of the top electrode). This last point was addressed by Sato's group (in another article by M. Ye at al., Low-Voltage-Driving Liquid Crystal Lens, Japanese Journal of Applied Physics 49 (2010) 100204, RAPID COMMUNICATION) by flipping upside-down the top substrate, by covering, the electrodes with 1 um SiO2 film and by using (as WCL) a highly resistive film of water-borne thermosetting paint (TWH-1, Mitsubishi Materials Electronic Chemicals). Finally, there are still some significant wave front (aberration) problems with this double voltage controlled scheme.
To resolve the remaining aberration (wave front) problems, LensVector 2 has introduced another (simpler) approach, where a transparent floating (non-connected) conductive layer (in general in the form of a disc) is introduced between the two cross oriented LC layers of Naumov's design, used in the “full” lens geometry, shown in FIG. 6. The presence of the floating conductive layer improves significantly (compared to Sato's and Naumov's designs) the wave front profile and the Modulation Transfer Function (MTF) of cameras using such lenses. In addition, the unique voltage required for the lens driving is very low and the device operates by frequency control.
Alternative approaches were proposed to resolve all three problems (poor WCL repeatability, high voltage and undesired wave front). One of them, proposed by N. Hashimoto, Liquid crystal optical element and method for manufacturing thereof, U.S. Pat. No. 7,619,713 B2, Nov. 17, 2009, is shown in FIG. 7. The basic difference of this design, compared to the Naumov's approach, is the absence of the WCL. In fact, Hashimoto proposes the use of optically transparent multiple concentric ring shaped electrodes (CRSE), which are interconnected via high resistivity “bridges” (the schematic side view is shown on the upper picture and the top view is shown in FIG. 8). This “resistively-bridged” structure plays the same role as the WCL in creating a voltage spatial profile over the aperture. The advantage of this approach is that we can adjust the individual resistivity values (R1, R2, etc.) of those bridges and obtain the desired wave front. Also, we need two small voltage V1 and V2 (applied to the center and to the periphery of the external ring shaped electrode; the TUE being grounded) to drive the lens. The manufacturing tolerances on the resistance trimming here can be expected to be very difficult to meet.
Another approach, proposed by Bos et al. Tunable electro-optic liquid crystal lenses and methods for forming the lenses, US Patent Application, Pub. No.: U.S. 2011/0025955 A1, Feb. 3, 2011, is shown in FIG. 9. Here, in addition to the resistive bridges (described by Hashimoto), we can also find a description of individually addressable CRSEs. Thus, in addition to manufacturing problems, the dynamic control of such lenses will be complicated (similar to LCDs).
Finally, an intermediate solution was proposed by V. Kato et al. Automatic focusing apparatus, US Patent Application, Pub. No. U.S. 2007/0268417 A1, Nov. 22, 2007, where there is a central DSE and a peripheral HPE, both connected to power supplies (with correspondingly voltages V1 and V2) while all intermediate CRSEs are connected via the resistive bridges to the DSE and HPE. This approach also suffers from manufacturability problems.
As we have already mentioned, in three above mentioned cases, we deal with either resistive bridges or individual control of concentric electrodes and thus the questions of wave front shape control and low voltage may be resolved in general. However, there are significant drawbacks in those approaches too. One of them is the abrupt variation of the field, particularly in the periphery of individual electrode segments. Thus in the area covered by one electrode segment, the potential is uniform, but there is an abrupt change between those segments. This requires very close electrode segments to minimize the impact of abrupt changes of the electrical potential. In addition, the relatively flat zones in the wave front will degrade the MTF of the camera and thus a very high number of such electrode segments is required. This, in turn increase the requirements on manufacturing precisions on those segments, on the resistive bridges and the dynamic control of voltage distributions of those structures also becomes extremely difficult to handle in practice.